11 and tends to be a stable disorder. These studies suggest that a demyelinating process is unlikely to cause SFN, as it would also affect large nerve fibers, which are not involved in SFN. Therefore, distal axonal loss or extraordinarily neuronal degeneration is more likely to be the underlying cause.4 The heterogeneous presentation within SFN has led to conceptual clinical phenotyping subsets of SFN that are presented in Figure 3. These clinical phenotypes may contribute to a better understanding of the pathophysiology and potential treatments. Four clinical phenotypes are suggested and distinguish: Small fiber sodium channel dysfunction Small fiber mediated painful neuropathy Small fiber mediated widespread pain Small fiber mediated autonomic dysfunction16 Figure 3 Roadmap: from symptoms to phenotypes and from phenotype to diagnostic method. Small fiber sodium channel dysfunction is caused by pathogenic mutations in the SCN9A, SCN10A and SCN11A genes, so genotyping is indicated. Skin biopsy, quantitative sensory testing (QST) and corneal confocal microscopy (CCM) can identify small fiber-mediated painful neuropathy and widespread pain. Autonomic dysfunction can be diagnosed with tests such as Sudoscan or the water immersion skin wrinkling test (WISW). 1 12 1
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